A fuel cell includes a membrane electrode assembly that has an anode electrode disposed on one surface of an electrolyte film and a cathode electrode disposed on the other surface of the electrolyte film. A fuel cell stack can be formed by laminating a plurality of unit cells, each including a gas passage layer and a separator that sandwich the membrane electrode assembly. Fuel gas that contains hydrogen is supplied to the anode electrode. The supplied fuel gas is converted into protons according to the electrochemical reaction expressed using the following formula (1). The generated protons pass through the electrolyte film and move toward the cathode electrode. Oxidant gas that contains oxygen is supplied to the other cathode electrode. The oxidant gas reacts with the protons supplied from the anode electrode. Water can be generated as a result of the electrochemical reaction expressed using the following formula (2). The electric energy can be obtained from the electrodes when the electrochemical reaction occurs on the electrolyte film side surfaces of a cell structure including the above-described pair of electrodes.Anode reaction: H2→2H++2e−  (1)Cathode reaction: 2H++2e−+(1/2)O2→H2O  (2)
The fuel gas and the oxidant gas supplied to respective electrodes pass through a fuel gas passage and an oxidant gas passage formed on a gas diffusion layer side surface of the separator. Each of the fuel gas passage and the oxidant gas passage is a groove having an uneven shape. The uneven grooves are integrally formed on the front and the back surfaces of the separator. One of the surfaces defines the fuel gas passage and the oxidant gas passage. The other surface defines a cooling water passage. More specifically, at the anode (negative electrode) side separator, the fuel gas passes through an uneven portion formed on the gas diffusion layer side. The cooling water passes through an uneven portion formed on the opposite side of the gas diffusion layer. Further, at the cathode (positive electrode) side separator, the oxidant gas passes through an uneven portion formed on the gas diffusion layer side. The cooling water passes through an uneven portion formed on the opposite side of the gas diffusion layer.
Patent Literature 1 discloses a fuel gas passage and an oxidant gas passage (hereinafter, collectively referred to as gas passage) that are connected via a manifold and a gas outlet/inlet portion formed outside a separator. The gas outlet/inlet portion has a circular shape, which can be formed through embossing finish. The embossing finish is intended to straighten the fuel gas and the oxidant gas (hereinafter, collectively referred to as “reactant gas”) and can adequately maintain the distributivity for the manifold and the gas passage.
The performance of the fuel cell is greatly dependent on the resistance value and the reactant gas fluidity in the separator. Therefore, reducing the contact resistance between the separator and the gas diffusion layer as well as reducing the reactant gas pressure loss in the separator are required. If the embossed finish separator discussed in Patent Literature 1 is laminated with a gas diffusion layer and a membrane electrode assembly, the separator is brought into contact with a neighboring gas diffusion layer at the embossed portion and the reactant gas flows in the area other than the embossed portion of the separator. Accordingly, to adequately maintain the contact resistance between the gas diffusion layer and the separator, it is desired to enlarge the contact area between the embossed portion and the gas diffusion layer. On the other hand, to adequately maintain the gas fluidity and suppress the gas pressure loss, it is desired to decrease the area of the embossed portion so that the gas passage can be secured sufficiently. More specifically, the contact resistance and the gas pressure in the gas outlet/inlet portion are in a trade-off relationship. Thus, the design of the gas outlet/inlet portion and the embossed portion is required to satisfy both of the above-described requirements.